This GM-Counter is build on 2 PCB’s. One is a standard high Voltage generating circuit, whilst the second is a Counter based on an ATMega16™ which also handles serial Communication with a host (Environmental Control).
The High Voltage generator is based on a 100 Hz Chopper, which is build around a ’555′ in combination with a standard Transformer and a Cascade to achieve Voltages from 400 to approx 900 V. (adjustable) The Regulation is just on-off (Burst) which will result in approx 1% Drift. This Circuit consumes about 20 mA at a 9 V (Battery). (more when starting up
Homebrew Geiger Müller Counter - [Link]